The Blues made a serious attempt in January for the midfielder.
"We were so close," Zola told Gazzetta dello Sport.
“He's strong and Sarri has always liked him, but then things fell through."
Zola gave the 22-year-old his senior debut in January 2015 as Cagliari coach.
"I gave Nicolo his debut at Parma in the Coppa Italia," he explained.
“At 17 he had the tools and character of the great player, but to play for a big club he must grow. His attitude isn't lacking."